1. Field of the Invention
The invention pertains to methods for fabricating devices which include multicomponent metal halide glasses, e.g., multicomponent metal halide glass optical fibers, as well as the resulting devices.
2. Art Background
Multicomponent metal halide glasses are glasses having compositions which include two or more metal halides (metal fluorides, metal chlorides, metal bromides or metal iodides). These glasses are of commercial/technological interest because, it is believed, they are potentially useful in devices employed for transmitting electromagnetic radiation, e.g., optical fiber. That is, one measure of the utility of any glass as an optical transmission medium is the total optical power loss produced by the glass. This loss is the sum of the extrinsic optical power losses (optical power losses due to impurities, compositional variations and defects) and intrinsic optical power losses (losses inherent to the glass, and due to factors other than impurities, compositional variations and defects) associated with the glass. In this regard, it is believed that multicomponent metal halide glasses exhibit minima in their intrinsic losses, at wavelengths between about 2 micrometers (.mu.m) and 10 .mu.m, which are far lower than the minimum loss exhibited by, for example, purified silica glass, currently employed in the manufacture of commercial optical fiber.
Presently available techniques for fabricating multicomponent metal halide glasses typically involve heating a mixture of two or more metal halides (usually in powder form) to a temperature equal to or greater than the melting temperature of the mixture, T.sub.m. The resulting melt is then cooled to a temperature which is equal to or less than the corresponding glass transition temperature, T.sub.g, to form a solid glass body.
As is known, cooling a melt from T.sub.m to T.sub.g necessitates passage through an intermediate temperature range where crystallites (particles found in a glass, produced by devitrification, and readily visible with an optical microscope) tend to form. Such crystallites are undesirable because they are responsible for, or increase, the extrinsic optical scattering associated with the resulting glass, and thus lead to increased optical power loss. Stated, alternatively, a measure of the total optical power loss produced by a glass is the ratio of the output optical power, Po, to the input optical power, Pi, which is given by the relation EQU P.sub.o /P.sub.i =10.sup.-.alpha.L/10. (1)
In Eq. (1), .alpha. denotes the optical loss coefficient of the glass in decibels per unit length, e.g., per kilometer, of the glass (dB/km), and L is the length of the glass traversed by the incident electromagnetic radiation in corresponding length units. Both optical absorption and optical scattering contribute to .alpha.. Significantly, the portion of .alpha. due to scattering alone, .alpha..sub.s, is readily measured as a function of the vacuum wavelength, .lambda., of the incident electromagnetic radiation, using conventional techniques. (Regarding these techniques see, e.g., Optical Fibre Communications, edited by the technical staff of CSELT (McGraw-Hill, N.Y., 1981), Chapter 3.) As is known, each such measured value of .alpha..sub.s consists of two components. The first component is due to intrinsic optical scattering (optical scattering inherent to the glass) and (because the contributions due to Raman and Brillioun scattering are negligible), when plotted as a function of .lambda., is well approximated by (the so-called Rayleigh form) B/.lambda..sup.4. Here, B is a material parameter, independent of .lambda., whose value is readily inferred from measurements of .alpha..sub. s at different wavelengths. (The intrinsic scattering is largely due to static density variations in the glass having dimensions which are small compared to the (vacuum) wavelength of the incident electromagnetic radiation.) The second component of .alpha..sub.s is due to extrinsic scattering (scattering which is not inherent to the glass), produced by density fluctuations in the glass having dimensions which are comparable to, or larger than, the (vacuum) wavelength of the incident radiation. For purposes of this disclosure, the second component is that portion of .alpha..sub.s which exceeds B/.lambda..sup.4. Typically, this excess is well approximated by the function C/.lambda..sup.2 +D where, as before, the values of C and D are readily inferred from measured values of .alpha..sub.s at different wavelengths. It is this second component of .alpha..sub.s which is due to, or increaed by, the presence of the crystallites.
Until recently, it was believed tht crystallites are avoided in any multicomponent halide glass provided the corresponding melt is quenched (cooled), through the temperature range where such crystallites tend to form, at a rate which is equal to or greater than a corresponding critical quench (cooling) rate, R.sub.C, whose value is determined using conventional differential scanning calorimetric (DSC) techniques. (Regarding these DSC techniques for determining R.sub.C see, e.g., T. Kanamori and S. Takahashi, Japanese Journal of Applied Physics, Vol. 24, p. L758(1985); and A. J. Bruce in Material Science Forum, Vol. 5 (Trans Tech Publication, Switzerland, 1985), p. 193.) Based upon this belief, at least three different techniques were developed for fabricating multicomponent metal halide glass optical fibers, capable of achieving quench rates equalt o or greater than the corresponding value of R.sub.C. In the first of these techniques, called built-in casting, a first melt is poured into a cylindrical mold, the central portion of the mold is poured out, and then a second melt is poured into the center of the mold. To achieve commercially significant lengths of fiber, the inner diameter of the mold is preferably 1 centimeter (cm) or larger. Upon cooling the two, concentric melts, the resulting solid glass (cylindrical) body (which necessarily has an outer diameter equal to or greater than 1 cm) constitutes an optical fiber preform, which is transformed into an optical fiber using conventional drawing techniques. (Regarding built-in casting see, e.g., S. Mitachi and T. Miyashita, Elec. Lett., Vol. 18, p. 170(1982).) Significantly, when using a mold having an inner diameter equal to or larger than 1 cm, and when forming a multicomponent metal fluoride glass, the built-in casting technique is capable of achieving a quench rate as high as about 10 Kelvins per second (K/sec), which is typically far higher than R.sub.C for any multicomponent metal halide glass.
In the second technique, called rotational casting, a first melt is poured into a vertically oriented, cylindrical mold which, after being horizontally oriented, is rotated about its longitudinal axis to force the melt (under the influence of the resulting centrifugal force) from the center of the mold to the inner surface of the mold. The mold is then vertically oriented, and a second melt is poured into the center of the mold. Again, conventional drawing techniques are used to convert the resulting cylindrical glass preform into an optical fiber. (Regarding rotational casting see, e.g., D. C. Tran et al, Elec. Lett., Vol. 18, p. 657(1982).)
In the third technique, called the double crucible technique, a first melt is poured into a first crucible, and a second melt is poured into a second crucible which encircles the first crucible, with the bottoms of the two crucibles converging to a common point. An optical fiber is drawn directly from this common point using conventional techniques. (Regarding the double crucible technique see, e.g., H. Tokiwa et al, Elec. Lett., Vol. 21, p. 1131(1985).)
Unfortunately, and despite the fact that the cooling rates always exceed the corresponding R.sub.C s, the above-described optical fiber fabrication techniques always yield optical fiber preforms, and thus optical fibers, which exhibit relatively large numbers of crystallites, readily seen with an optical microscope. As a consequence, at the minimum loss wavelengths for these optical fibers, as determined via the conventional cutback technique, the portion of .alpha..sub.s due to extrinsic scattering always exceeds 0.1dB/km. (Regarding the cutback technique see, e.g., D. Marcuse, Principles of Optical Fiber Measurements (Academic Press, N.Y., 1981), chapter 5.)
Because the above-described belief concerning critical cooling rates has proven to be meaningless, those artisans engaged in the fabrication of devices which include multicomponent halide glasses have, to date, been stymied in their efforts to prevent the formation of undesirable crystallites in these devices.
Thus, those engaged in the fabrication of devices which include multicomponent halide glasses have sought, thus far without success, techniques for fabricating such devices which avoid undesirable crystallite formation.